Subsea well operations include the installation on the sea floor of subsea well apparatus which may include wellheads, production modules, and control modules. In some subsea installations, wellhead assemblies, production head assemblies and control head assemblies may be combined into a subsea station, such as that shown and described in copending application Ser. No. 743,586, filed Nov. 12, 1976 owned by a common assignee. After such subsea installations have been made, it is often necessary to return to the subsea installation to provide service and maintenance operations which may include the recovery or retrieval of certain well equipment and the installation of other well equipment.
Reentry to such a subsea installation or station has been usually accomplished by providing a plurality of guide lines. In prior proposed systems, such guide lines were supported below the surface of the water by suitable buoys or were permitted to fall and lay along the sea floor. When guide lines were supported by buoys, such lines were subjected to twisting by ocean currents and were often difficult to untangle for use. In addition, such upstanding lines presented a hazard to other subsea operations in the vicinity. When the guide lines were left on the sea floor, they were often difficult to retrieve in proper untangled condition.
It has, therefore, been proposed that reentry to such a subsea station be accomplished by attaching a single guide line to the station by automatic and remote control means, which could seek an upstanding guide post fixed to the subsea installation, automatically connect one end of a single guide line to such post, and then use the single guide line for guide means to lower well equipment of other sets of guide lines upon which well equipment could be lowered to the subsea station. Such fixed guide posts at a subsea station were exposed because of their length, were subject to damage, and were subject to constant exposure to the sea water whereby its condition after a long period of time made the guide post unsuitable for use.
Such prior conditions and prior proposed solutions to reentry to a subsea well apparatus are shown and described in U.S. Pat. Nos. 3,493,043; 3,503,442; 3,513,909 and 3,605,884, which show installation and use of a single or mono guide line apparatus and method.